theorem :: constants not overloaded if monotone
  S is monotone & the_arity_of o1 = {} & o1 ~= o2 & the_arity_of o2 = {}
  implies o1=o2
proof
  assume that
A1: S is monotone and
A2: the_arity_of o1 = {} & o1 ~= o2 & the_arity_of o2 = {};
  the_result_sort_of o1 <= the_result_sort_of o2 & the_result_sort_of o2
  <= the_result_sort_of o1 by A1,A2,Def7;
  then the_result_sort_of o1 = the_result_sort_of o2 by ORDERS_2:2;
  hence thesis by A2,Def3;
end;
